This disclosure relates generally to a chemical reactor system employing a rigid packing material that provides selected support to a packed catalyst bed. More particularly, the disclosure relates to the production of bisphenols in a downflow mode through a bed of cross-linked ion exchange resin catalyst interspersed with and supported by a randomly distributed, substantially inert packing material.
Ensuring that reacting species achieve optimal physical contact is a difficult challenge in chemical reactor design. If done improperly, numerous undesired byproducts and an abundance of unreacted reactants can seriously impact the economics of the system. The reactor type, reactant and production diffusion, pressure effects, and other factors must all be considered in selecting or fabricating a reactor system best suited for use in a given reaction.
Reactor residence time and reaction conditions such as temperature impact the percentage of atomic or molecular collisions and thus affect yield, throughput, and selectivity. Pressure becomes important if the differential pressure compresses and deforms the spherical catalyst beads causing a reduction in liquid throughput as a result of pressure drop limitations.
In reactors having packed beds, fluid flow properties oftentimes suffer significant disruption of flow, or “channeling”. This is especially evident in upflow mode. Channeling is a condition that may be caused by an inadequate pressure differential over the height of the bed through which the fluid flows, and typically results from settling of the bed components coupled with too little differential pressure applied over the height of the bed. If the bed components include a catalyst or a similar particulate treatment agent that allows for the random flow of fluid through the bed, portions of the bed may be short-circuited and not contacted by the fluid in a uniform and consistent manner. Such a condition can lead to the incomplete treatment or incomplete chemical reaction of the charged reactants. This, in turn, can result in the premature disposal of the catalyst or treatment particles, which translates into loss of part of the value of the catalyst.
The amount of channeling that occurs can be related to reactor geometry and type; fluid dynamics of the reactants, intermediates and products produced in the reactor; and other factors. In some processes optimizing the production of products by adjusting these parameters is easily understood and straightforward. In others, the relationships are not quite as clear. The employment of carefully selected catalysts complicates reactor design and reaction control. For example, U.S. Pat. No. 5,395,857 proposes that in the production of bisphenol A (BPA) in a downflow reactor, the degree of crosslinking of certain ion exchange resin catalysts directly affects the physical performance of the process as well as the reactivity and selectivity of the reaction. This patent found that the hydraulic impact attributable to catalyst particle shape and the compression of a catalyst bed due to pressure can be lessened by using a two-layer catalyst in which one of the layers exhibits a 2% or lower degree of crosslinking. The process is directed to increasing the volume and time yield of fixed bed reactors. This design permits a higher throughput and production due to an overall greater bed rigidity, while gaining the important aspects of the lower 2% crosslinked catalyst in the top portion where the bulk of the conversion of the reactants occur. The combined catalyst bed proposed in U.S. Pat. No. 5,395,857 has greater selectivity and activity than others and would be desirable because resin-based catalysts with greater degrees of crosslinking are more subject to deactivation and thus become inactive more readily. For example, in downflow processes the potential for catalyst bed collapse at high flow rates because of the low degree of crosslinking and the effects that this has on the physical properties of the catalyst must be considered and ways of reducing or eliminating this problem would be advantageous.
In addition, because of channeling and the resultant inefficient contact of the reactants with the catalyst, the operation of packed bed reactors is often significantly hindered. In particular, operations in which significant amounts of channeling occur generally result in low product yield, premature replacement of catalyst beds, and inefficient use of the reactants. This results not only in the cost of new catalyst, but also loss of production during outages, the logistical costs of replacement, used catalyst disposal costs, and recovery and recycling of the reactants. Furthermore, a significant financial burden may be realized as a result of costs associated with efforts to improve catalyst technology. Such costs involve the development of alternate reactor geometries but do not address the problem of existing reactors which have less favorable geometric features.
With respect to the downflow configuration of reactants into a reactor having a fixed catalyst bed, depending upon selection of the catalyst, the compression due to pressure within the catalyst bed can result in a significant hindrance of the physical performance of the process, as well as the reactivity and selectivity of the reaction. Attempts have been made to utilize a catalyst having a more robust architecture in order to minimize the compression of catalyst particles. This often, however, results in catalysts which are less active, are less selective, or have a shorter lifetime.
Furthermore, there is a direct relationship between the rigidity of catalyst particles and the expected active lifetime of those particles. Particles having an open effective pore structure, which is characteristic of catalysts having a small degree of crosslinking, and catalysts having a less rigid structure, can be expected to result in the reduction or elimination of fouling of the resin catalyst with tar-like molecules that block access to the active acid sites. On the other hand, particles having a less open effective pore structure and greater rigidity would resist compression better, but they may lead to the premature deactivation of the catalyst resin through fouling, thereby resulting in increased costs.
While existing reactor geometries and catalysts are suitable for their intended purposes, there still remains a need for improvements, particularly regarding the effectiveness of the reaction and the catalyst itself in a downflow reactor. Therefore, a need exists for a reactor system that allows the full potential of the selected resin catalyst to be exploited for example, by mitigating associated hydraulic limitations.